In conventional telephone exchanges, the subscriber fluctuation is only small and is determined essentially by changes of the subscribers locations. As a result, the fluctuation of subscriber data in the associated subscriber data bases is also small. In mobile-radio systems, this is completely different. Although the data of mobile-telephone subscribers is stored where the mobile-radio stations are registered, namely in a so-called home location register (HLR), it is not generally to be expected that a mobile-telephone subscriber is constantly in the service area of the same switching center in which his subscriber data is stored. It is rather conceivable that a mobile-telephone subscriber who is far away from his residence, possibly in another country, wishes to set up a telephone connection with another, likewise travelling mobile-radio subscriber. To permit such connections, a number of mobile switching centers (MSC) are distributed over the area in which the mobile-radio system is to be operated. Unlike data bases of conventional telephone exchanges, the data bases of mobile switching centers, so-called visitor location registers (VLRs), must work properly even with high subscriber fluctuation.
DE 40 21 632 A1 proposes a method of filing or finding subscriber data records in a subscriber data base with high subscriber fluctuation. In that method, a data record in a memory area is unambiguously assigned to an N-digit subscriber number IMSI (International Mobile Station Identity) with the aid of an algorithm. This permits access to a VLR subscriber data record. The algorithm generates virtual indices V0 through V4, with the aid of which the data record in a memory area is determined. A 15-digit IMSI can be mapped by the algorithm into independent values V0, . . . , V4 as follows: ##EQU1## where: V0 through V4=the digits of the virtual indices
D(0) through D(14)=the digits of the subscriber number PA1 A%B=A modulo B PA1 VC=a correcting quantity calculated from all 15 digits of the IMSI.
For repeated accesses, the Temporary Mobile Subscriber Identity Signal formed with the aid of this algorithm offers a more appropriate "key", since the IMSI consists of 15 digits; in addition, in case of unencoded accesses (radio link), the subscriber's identity cannot be kept anonymous with the IMSI.
It is, therefore, the object of the invention to provide a method wherein each subscriber is assigned a TMSI-in a VLR. With the aid of this TMSI, communications are to be established between VLR and subscriber. This object is attained by a method of determining a temporary mobile subscriber identity signal (TMSI) in a telecommunication network comprising a subscriber data base on the basis of a permanent mobile station identity signal (IMSI) permanently assigned to a subscriber, characterized by the following steps: determining a memory area index signal (MR) and a data-record index signal (DS-IX) in the subscriber data base from the permanent mobile station identity signal (IMSI), the memory area index signal (MR) being a first part of the temporary mobile subscriber identity signal (TMSI); encoding the data-record index signal (DS-IX) by adding an addition parameter signal (ADD-PAR) which is dependent on a generation parameter index signal (GP-IX) to form a temporary subscriber index signal (TMSI-IX), providing the temporary subscriber index signal (TMSI-IX) as a second part of the temporary mobile subscriber identity signal (TMSI), providing the generation parameter index signal (GP-IX) as a third part of the temporary mobile subscriber identity digital signal (TMSI), and providing additional information signal (RC, CD) about the status of the subscriber data base as fourth and fifth parts of the temporary mobile subscriber identity signal (TMSI). Further advantageous features of the invention are that the generation parameter index signal (GP-IX) address a generation parameter table (GEN-TAB), thus generating the addition parameter signal (ADD-PAR); that to generate a new temporary subscriber identity signal (TMSI), the generation parameter index signal (GP-IX) is incremented; that the values of the generation parameter table (GEN-TAB) are changed at time intervals; that the information on the status of the subscriber data base is contained in a recovery code signal (RC) and in a configuration discriminator signal (CD); that the recovery code signal (RC) is incremented when the data of a memory area (MR) is lost; that the configuration discriminator signal (CD) is incremented when data changes its location; that the configuration discriminator signal (CD) can be incremented when the recovery code signal (RC) overflows to 0 during incrementation; that the TMSI is a digital signal which consists of 32 bits; and that the TMSI of a subscriber, together with the subscriber signal, such as the International Mobile Station Identity signal (IMSI) is permanently assigned to said subscriber, and entered into a table (EXCL-TAB) before being deleted from the data base. The method according to the invention users a four-byte-long TMSI. Anonymity of the subscriber is ensured by frequent changes of elements of the TMSI.
One advantage of the method to the invention lies in the unambiguous addressing of up to 10.sup.6 subscribers. Another advantage is that any confusion of data records is excluded even if a data record in the data base is deleted or if all data records of a memory area MR are lost.
A further advantage of the method lies in the face that data-base extensions and reconfigurations of data are supported. In addition, losses of data in the memory are prevented.